CIE A Level Physics (9702) 2019-2021

Revision Notes

27.1.1 The Photon

The Particle Nature of Light

  • In classical wave theory, electromagnetic (EM) radiation is assumed to behave as a wave
  • This is demonstrated by the fact EM radiation exhibits phenomena such as diffraction and interference
  • However, experiments from the last century, such as the photoelectric effect and atomic line spectra, can only be explained if EM radiation is assumed to behave as particles
  • These experiments have formed the basis of quantum theory, which will be explored in detail in this section

Calculating Photon Energy

  • The energy of a photon can be calculated using the formula:

E = hf

  • Using the wave equation, energy can also be equal to:

3. Calculating Photon Energy equation 1

  • Where:
    • E = energy of the photon (J)
    • h = Planck's constant (J s)
    • c = the speed of light (m s-1)
    • f = frequency in Hertz (Hz)
    • λ = wavelength (m)
  • This equation tells us:
    • The higher the frequency of EM radiation, the higher the energy of the photon
    • The energy of a photon is inversely proportional to the wavelength
    • A long-wavelength photon of light has a higher energy than a shorter-wavelength photon

 

Worked example: Calculating photon energy

Calculating_Photon_Energy_Question, downloadable AS & A Level Physics revision notes

Step 1:            Write down the known quantities

Wavelength, λ = 490 nm = 490 × 10-9 m

Power, P = 3.6 mW = 3.6 × 10-3 W

Time, t = 2.0 s

Step 2:            Write the equations for wave speed and photon energy

Calculating Photon Energy Worked Example equation 1

Calculating Photon Energy Worked Example equation 2

Step 3:            Calculate the energy of one photon

Calculating Photon Energy Worked Example equation 3

Step 4:            Calculate the number of photons hitting the surface every second

Calculating Photon Energy Worked Example equation 4

Step 5:            Calculate the number of photons that hit the surface in 2 s

(8.9 × 1015) × 2 = 1.8 × 1016

Exam Tip

The values of Planck’s constant and the speed of light will always be given to you in an exam, however, it helps to memorise them to speed up calculation questions!

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Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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